The present invention relates to a process for monitoring the quality of latices of halogen-containing vinyl polymers. It relates more particularly to a process for monitoring the quality of latices of such polymers during the radical polymerization of halogen-containing vinyl monomers in aqueous emulsion. It also relates to a device for monitoring the quality of the aqueous emulsions.
Radical polymerization in aqueous emulsion, typically using, besides an aqueous polymerization medium, emulsifying agents and radical initiators, constitutes a polymerization technique which is particularly well suited to the manufacture of aqueous emulsions of halogen-containing vinyl polymers, usually called latices, such as, for example, latices of polyvinyl chloride or of vinylidene chloride copolymers. The latices thus produced contain, ultimately, elementary polymer particles which have very small mean diameters that can range from approximately 10 to approximately 4000 nm (nanometers). For some applications, such as, for example, in the field of paints or generally of coatings, these latices find an application directly without preliminary isolation of the polymers from their polymerization medium. In other applications, such as, for example, PVC-based plastisols, the polymers are isolated from their polymerization medium and dried before being used. In all cases the quality of the manufactured products is dependent in particular on the mean diameter of the polymeric elementary particles of the latex and the diameter distribution of the particles.
This is why it has been found desirable to have at one""s disposal a process for effective and rapid monitoring of the quality of the latices during the polymerization in aqueous emulsion in order to have the ability of ensuring a very high reproducibility of the quality of the latices and of the polymers produced. Monitoring the progress of a polymerization, by means of measurements reflecting the properties of the polymer being formed, makes it possible, in fact, to control the polymerization conditions so as to produce polymers which have predetermined properties with an improved reproducibility.
It has already been proposed to monitor properties characterizing the molecular structure of polymers, such as the viscosity index or the density of the polymers, by means of the measurement of the absorption spectrum in the near infrared (xe2x80x9cNIRxe2x80x9d hereinafter) region on samples of polymers taken during polymerization and freed beforehand from their polymerization medium and, in particular, on olefin polymers in powder form (Patent Application EP-A-0 328 826). The process in question permits control of the polymerization by modifying the polymerization conditions when a predetermined difference appears between the measured properties of the isolated polymer and those which it is desired to attain. Nevertheless, the process according to document EP-A-0 328 826 requires regular sampling and analysis of samples of polymer which has been separated beforehand from its polymerization medium and is therefore not suitable for an in-situ monitoring during polymerization.
The measurement has furthermore been described (with a view to a calibration) of the spectrum transflectance in the near infrared region, ranging from 900 to 1200 nm, on the light transmitted through latices of copolymers of styrene and of methyl acrylate, with a solids content of 30% by weight, containing polymer particles whose mean diameter is approximately 60 to 80 nm (Paul D. Gossen et al., Applied Spectroscopy, vol. 47, No. 11, 1993, pp. 1852-1870). These measurements have made it possible to establish some correlations between the transflectance at specific wavelengths and some properties of the latices of copolymers of styrene and of methyl acrylate, such as the mean particle diameter and the solids content of the latices. However, this xe2x80x9coff-linexe2x80x9d calibration technique using the light transmitted through the latex is described as not being suitable for latices containing polymer particles whose mean diameters are from approximately 150 to 200 nm (and more). It cannot therefore be adapted to the monitoring of industrial latices whose concentration can exceed 50% at the end of polymerization, or to the determination of the mean diameters and more generally of the diameter distribution of the polymer particles in aqueous emulsions consisting of one or a number of classes of particles whose final mean diameter is generally greater than approximately 200 nm.
The present invention is aimed at providing a process for monitoring the quality of latices of halogencontaining vinyl polymers which does not exhibit any of the abovementioned disadvantages.